(1) Field of the Invention
The present invention relates to a method and to a device for bonding a metal fairing to the leading edge of an aircraft airfoil, e.g. a rotorcraft rotor blade.
(2) Description of Related Art
The leading edge of a rotorcraft rotor blade is protected against impacts, e.g. by a U-section metal fairing bonded to the structure of the blade, or indeed by a fairing made from composite materials as taught in document EP 0 529 917 A, for example. The metal fairing thus protects the leading edge of the blade and also protects portions of the suction and pressure sides of the blade that are directly adjacent to the leading edge.
When the metal fairing is damaged by impacts, or even by an erosion phenomenon, it is appropriate to replace it with a new metal fairing so as to ensure that the blade will continue to be properly protected.
Furthermore, it can be necessary to remove the leading edge in order to repair the structure of the blade, particularly if the blade is made of composite material and surface layers of the structure are delaminating. Once the repair has been made, the leading edge needs to be put back into place.
A device is known that enables the metal fairing for the leading edge of a blade to be stuck back into position by using an adhesive that is placed between the metal fairing and the structure of the blade.
That device makes use of heater tooling that delivers the energy needed to polymerize an adhesive so as to secure the metal fairing to the structure of the blade, the adhesive being placed between the metal fairing and the structure of the blade. The heater tooling is constituted by a mat that is placed on the outside surface of the metal fairing.
The mat contains a heater resistance that is embedded in a layer of silicone. On being powered electrically, the heater resistance heats up, thereby enabling the metal fairing to be bonded to the structure of the blade. The heater resistance then heats the metal fairing by conduction, which fairing in turn transmits heat to the adhesive, thereby causing it to polymerize.
That device operates properly. Nevertheless, it presents a few drawbacks.
The first drawback is a consequence of using an electrical resistance. All of the elements surrounded by the heating mat are then heated by the mat, i.e. simultaneously the metal fairing, the adhesive, and the structure. That leads to energy consumption that is not necessarily optimized.
Similarly, a second drawback lies in an edge effect. A large amount of heat is lost by conduction from the edges of the device, which means that the device operates non-uniformly. Certain zones of the assembly comprising the metal fairing and the blade structure are thus heated to a smaller extent than others, thereby ending up with over-consumption of energy in order to compensate for the losses.
A third drawback is a consequence of the temperature rise times of the heater resistance, with rates of rise being very slow, of the order of 1.5 degrees Celsius per minute (° C./min). The duration of the cycle for heating the metal fairing can then easily amount to eight hours, which is penalizing industrially speaking from an economic point of view.
Furthermore, the airfoil is subjected to large temperature stresses over a long period due to the slow rises in temperature, and that can degrade the airfoil.
Finally, fabricating the mat is very lengthy, and therefore expensive and incompatible with urgent industrial demand, as can happen unfortunately when a manufacturer in possession of a heater mat that is in poor condition needs to repair a blade.
Document WO 01/30116 discloses a bonding device that includes induction heater means for melting an adhesive. Nevertheless, it would appear to be difficult to apply that bonding device to the metal fairing of the leading edge of an airfoil.